The present application claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2000-370704, filed on Dec. 5, 2000. The contents of that application are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a steering unit of a vehicle, and more particularly to a control of the steering unit.
2. Description of Related Art
In many vehicles wheels are provided, and steering is performed by changing directions of some of the wheels that are provided therein. Moreover, in steering, a power-steering unit which generates a force for assisting a force from a driver is also known.
Japanese Patent Publication No. 2000-62634 discloses an art related to a power steering unit, for always realizing constant steering reaction force characteristics by generating a target steering force with respect to a steering angle, and controlling an actual steering force so that it responds to the target steering force. Moreover, Japanese Patent Publication No. 2000-142434 discloses an art related to a power-steering unit, which performs control adapted to an increase in viscosity resulting from a drop in an oil temperature, by estimating an oil temperature in the steering unit and performing a control based on the thus estimated oil temperature.
Information on a reaction force against a steering operation performed by a driver is essential for the driver to know a road surface condition or the like. However; in the steering unit disclosed in the first of the above-mentioned patent applications, a control is performed such that the steering reaction force characteristics become constant without a reaction force from the road surface being incorporated. This causes a problem that it is difficult to convey information on the road surface condition of to the driver. Further, in the steering unit disclosed in the second of the above-mentioned patent applications, though it is possible to estimate a change in viscosity by temperature, it is not possible to respond to a change in the steering reaction force based on a secular change or a variation among individual steering units or the like.
It is therefore a first object of the present invention to calculate a drag generated in the steering unit which acts against the steering operation among forces acting on the steering unit. It is a second object of the present invention to provide a power-steering unit which generates adequate assistant force for the drag.
A steering unit of a vehicle is acted on by a force accompanying a steering operation of a driver, an assistant force for assisting the steering operation, a force input from outside of the vehicle, such as a road surface, to wheels subject to steering, and a force generated by a friction generated in the steering operation. If the above-mentioned forces acting on the steering unit and a force required for steering are obtained, an external force that is acting on the steering unit for other purposes can be obtained. The external force includes the above-mentioned outside force input from the outside of the vehicle and the above-mentioned internal drag generated in the steering unit The internal drag can be obtained by separating these two forces.
Further, the present invention can provide a steering unit that controls the assistant force based on the internal drag, and conveys, without loss, the information input from the road surface to the driver.
When separating the external force into the outside force and the internal drag, the external force and the outside force can be calculated by setting functions for defining them, and obtaining coefficients and constants of these functions. The function of the outside force is a function of a predetermined physical quantity indicating an amount of the steering operation, and the function of the internal drag is a function of a predetermined physical quantity indicating the speed of the steering operation. The above-mentioned functions can be defined by repeatedly detecting and calculating the external force and these predetermined physical quantities. Thus, the external force and the internal drag at present can be calculated from these functions.
Force acting on the steering unit can be calculated by storing the characteristics on motion of the steering unit beforehand, detecting predetermined physical quantities indicating the motion state of the steering unit, and applying them to the characteristics that were stored beforehand Characteristics related to the motion of the steering unit are expressed, for example, by an equation of motion related to a movable portion of the steering unit. Thus, the above-mentioned force acting at present can be obtained by repeatedly applying force acting on the steering unit, the operation amount, and the operation speed.
Further, the outside force can be calculated by storing the characteristics on motion of the vehicle beforehand, detecting predetermined physical quantities indicating the motion state of vehicle, and applying them to the characteristics that are stored as mentioned above. Characteristics related to motion of the vehicle are expressed by an equation of motion of the vehicle, for example. Thus, the outside force at present, the force input to the steering unit from the outside of the vehicle can be obtained, by repeatedly applying a force acting on the vehicle, a speed in a plane which intersects perpendicularly with the yaw axis of the vehicle, and the rotational speed around the yaw axis.